Can I use my voip cable modem instead of an ATA or FXO card?

I recently turned off my phone service through my cable company, but I still have a voip cable modem (Thomson DHG575). Can I use Asterisk to help me communicate with my old phones via Google Voice utilizing the voip cable modem? The datasheet for the modem claims that the G.729a codecs are included (in addition to a host of other acronyms that make my head spin). I just want to be able to use my old wireless phones with my Google Voice account (and/or Skype etc) without having to buy a new appliance. Is there something programmed into the modem to keep me from accessing the phone lines if I’m not paying for the phone service through the cable company anymore?

Modems that are provided by the ISPs usually have dual layer access, which limits the “normal user” from accessing more advanced settings - one of them is VoIP. If you can not see any VoIP / SIP account settings in the cable modem WebGUI, you can not use your cable modem for your requirement, since you do not have access to the needed settings.

Is there any way to unlock a voip cable modem and use with other voip phone systems? I’ve got a Vonage voip modem that I don’t use anymore but would like to be able to. Is there a code or admin only access to be able to change or mess with the voip settings?